D7 'Aggregate and Compress CSS Files' - live site losing styles???

Everytime I select 'Aggregate and Compress CSS Files' in D7 Performance settings on my live site I'm instantly losing my styles. However on my local machine this does not happen. Interestingly this only happens on my live site when I have my sub-theme of Bartik selected. If I use the default Bartik theme there is no issue. I've searched Google and found a plethora of possibilities but I still haven't found the solution. If anyone could advise it would greatly appreciated.

I'd done that before and was still have the sane problems. Oddly though, I've set the aggregate settings on the default Bartik and then changed to the sub-theme and now there's no problem?? I'm not sure how this has worked but it's sorted it. Thanks anyway :)

Your post isn't really applicable to the High-Performance group, but a frequent cause of your symptoms is invalid CSS. When the CSS compressor tries to combine all the CSS files it sometimes chokes on invalid CSS. Try running your CSS through a validation tool.

Before changing ownership and/or permissions, check your error logs! If PHP can't write to a destination, then it will produce an error.

You can also check Drupal's logging to see if there are 404s on the style sheets.

If there's nothing there, then fire up developer tools in Chrome or Firebug in Firefox and check the network tab to make sure the stylesheets are being requested. Click on them to make sure that content is being returned.

If all that is good, then grep one of the compressed stylesheets for a rule that you have in your sub-theme to make sure that file is being aggregated.

As I replied in the second post oddly it started working again but I've just added the CDN module, set it all up correctly, got it working with the exception that - again- it immediately losses all the 'front end' styles (note the admin functions retain their styles). I've even tested it on a very basic site of ours (see www.cerebraluk.com) that doesn't even use any sub-themes (just Bartik) and it has the same problem - add CDN and it loses all the 'front-end' styles.

Thanks David. I had seen this and have tried all the solution to no avail. Particularly the NGINX server reference in:

3 "For others having issues with css in IE when aggregate is on - take a look at http://web-sniffer.net/ and look at your http headers when aggregate and compress is off. If the headers are still showing the content is being compressed, you probably have the same issue."

This was not the issue on testing as the sites with 'aggregate and compress is off' were not showing the content as being compressed.

This is now fixed. Thanks everyone for their help. In a nutshell the styles were getting lost with 'Far Future expiration' turned on because I think I'm using an NGINX config (turned off and it worked).